revsets: add revset yielding merge commits

lib/src/revset.rs

@@ -15,6 +15,7 @@
 use std::cmp::{Ordering, Reverse};
 use std::collections::HashSet;
 use std::iter::Peekable;
+use std::ops::Range;
 use std::rc::Rc;
 
 use pest::iterators::Pairs;
@@ -122,6 +123,10 @@ pub enum RevsetExpression {
     PublicHeads,
     GitRefs,
     NonObsoleteHeads(Rc<RevsetExpression>),
+    ParentCount {
+        candidates: Rc<RevsetExpression>,
+        parent_count_range: Range<u32>,
+    },
     Description {
         needle: String,
         candidates: Rc<RevsetExpression>,
@@ -422,6 +427,25 @@ fn parse_function_expression(
                 })
             }
         }
+        "merges" => {
+            if arg_count > 1 {
+                return Err(RevsetParseError::InvalidFunctionArguments {
+                    name,
+                    message: "Expected 0 or 1 arguments".to_string(),
+                });
+            }
+            let candidates = if arg_count == 0 {
+                RevsetExpression::non_obsolete_commits()
+            } else {
+                Rc::new(parse_expression_rule(
+                    argument_pairs.next().unwrap().into_inner(),
+                )?)
+            };
+            Ok(RevsetExpression::ParentCount {
+                candidates,
+                parent_count_range: 2..u32::MAX,
+            })
+        }
         "description" => {
             if !(1..=2).contains(&arg_count) {
                 return Err(RevsetParseError::InvalidFunctionArguments {
@@ -586,6 +610,39 @@ impl<'repo> Iterator for ChildrenRevsetIterator<'_, 'repo> {
     }
 }
 
+// TODO: Generalize this to be take a predicate to apply to an &IndexEntry.
+struct ParentCountRevset<'revset, 'repo: 'revset> {
+    candidates: Box<dyn Revset<'repo> + 'revset>,
+    parent_count_range: Range<u32>,
+}
+
+impl<'repo> Revset<'repo> for ParentCountRevset<'_, 'repo> {
+    fn iter<'revset>(&'revset self) -> RevsetIterator<'revset, 'repo> {
+        RevsetIterator::new(Box::new(ParentCountRevsetIterator {
+            iter: self.candidates.iter(),
+            parent_count_range: self.parent_count_range.clone(),
+        }))
+    }
+}
+
+struct ParentCountRevsetIterator<'revset, 'repo> {
+    iter: RevsetIterator<'revset, 'repo>,
+    parent_count_range: Range<u32>,
+}
+
+impl<'revset, 'repo> Iterator for ParentCountRevsetIterator<'revset, 'repo> {
+    type Item = IndexEntry<'repo>;
+
+    fn next(&mut self) -> Option<Self::Item> {
+        while let Some(next) = self.iter.next() {
+            if self.parent_count_range.contains(&next.num_parents()) {
+                return Some(next);
+            }
+        }
+        None
+    }
+}
+
 struct UnionRevset<'revset, 'repo: 'revset> {
     set1: Box<dyn Revset<'repo> + 'revset>,
     set2: Box<dyn Revset<'repo> + 'revset>,
@@ -808,6 +865,16 @@ pub fn evaluate_expression<'revset, 'repo: 'revset>(
             let base_set = evaluate_expression(repo, base_expression.as_ref())?;
             Ok(non_obsolete_heads(repo, base_set))
         }
+        RevsetExpression::ParentCount {
+            candidates,
+            parent_count_range,
+        } => {
+            let candidates = evaluate_expression(repo, candidates.as_ref())?;
+            Ok(Box::new(ParentCountRevset {
+                candidates,
+                parent_count_range: parent_count_range.clone(),
+            }))
+        }
         RevsetExpression::PublicHeads => {
             let index = repo.index();
             let heads = repo.view().public_heads();

lib/tests/test_revset.rs

@@ -769,6 +769,38 @@ fn test_evaluate_expression_obsolete(use_git: bool) {
     tx.discard();
 }
 
+#[test_case(false ; "local store")]
+#[test_case(true ; "git store")]
+fn test_evaluate_expression_merges(use_git: bool) {
+    let settings = testutils::user_settings();
+    let (_temp_dir, repo) = testutils::init_repo(&settings, use_git);
+
+    let mut tx = repo.start_transaction("test");
+    let mut_repo = tx.mut_repo();
+    let mut graph_builder = CommitGraphBuilder::new(&settings, mut_repo);
+    let commit1 = graph_builder.initial_commit();
+    let commit2 = graph_builder.initial_commit();
+    let commit3 = graph_builder.initial_commit();
+    let commit4 = graph_builder.commit_with_parents(&[&commit1, &commit2]);
+    let commit5 = graph_builder.commit_with_parents(&[&commit1, &commit2, &commit3]);
+
+    // Finds all merges by default
+    assert_eq!(
+        resolve_commit_ids(mut_repo.as_repo_ref(), "merges()"),
+        vec![commit5.id().clone(), commit4.id().clone(),]
+    );
+    // Searches only among candidates if specified
+    assert_eq!(
+        resolve_commit_ids(
+            mut_repo.as_repo_ref(),
+            &format!("merges(,,{})", commit5.id().hex())
+        ),
+        vec![commit5.id().clone()]
+    );
+
+    tx.discard();
+}
+
 #[test_case(false ; "local store")]
 #[test_case(true ; "git store")]
 fn test_evaluate_expression_description(use_git: bool) {
@@ -804,7 +836,7 @@ fn test_evaluate_expression_description(use_git: bool) {
         resolve_commit_ids(mut_repo.as_repo_ref(), "description(\"commit 2\")"),
         vec![commit2.id().clone()]
     );
-    // Searches only in given base set if specified
+    // Searches only among candidates if specified
     assert_eq!(
         resolve_commit_ids(
             mut_repo.as_repo_ref(),